Distributed denial of service and SQL injection are the main types of attack discussed on hacking forums, according to new research from security vendor Imperva.

Underground discussion forums are an important piece in the cybercriminal ecosystem. They offer a place for hackers to sell and exchange information, software tools, exploits, services and other illegal goods.

“Forums are the cornerstone of hacking — they are used by hackers for training, communications, collaboration, recruitment, commerce and even social interaction,” Imperva stressed.

The company’s researchers have recently analyzed discussions going back several years from HackForums.net, one of the largest hacker forums with over 220,000 registered members. Their effort was aimed at determining the most common attack targets, what business trends can be observed, and what directions hackers are leaning toward.

As far as attack popularity goes, the analysts determined that DDoS was mentioned in 22 percent of discussions. SQL injection, a technique commonly used to compromise websites, is the second most frequently discussed attack method, being at the center of 19 percent of conversations.

Unsurprisingly, with a 16 percent discussion occurrence rate, spam is the third most favorite attack type according to Imperva’s content analysis. That’s probably because it is one of the primary methods of generating illegal income.

Zero-day exploits make up 10 percent of attack discussions on the forum, however, Microsoft’s latest Security Intelligence Report (SIR) claims that this type of exploit is used in less than 1 percent of real-world compromises.

Forums are also an important learning tool for new hackers — Imperva determined that up to a quarter of discussions fall into the beginner hacking category. Another 25 percent of conversations involved hacking tools and programs, while a fifth mentioned Web and forum hacking.

One trend observed by Imperva’s researchers was that mobile hacking is increasingly popular. This is also reflected in real-world attack statistics and reports from other vendors. iPhone hacking in particular accounted for half of conversations on this topic.

Overall, discussions about hacking have increased more than 150 percent over the last four years. “We think the growth in hacker forum activity helps explain that, along with automated hacking, there are simply more hackers causing more breaches,” Imperva concluded.